Eddy current sensors are well known in the art. These devices are used in a variety of situations where non-contact measurements for parameters such as separation or electrical resistance are desired. Eddy current sensors have been utilized to measure displacement, electrical properties and other physical characteristics of material such as thickness or flaws that modify the apparent electrical characteristics. One such application is the measurement of runout in magnetic steel rollers. Eddy current sensor systems have been used to measure the runout in the rollers but accuracy is limited because of sensitivity to permeability changes that are typical in such rollers. Thickness measurement of aluminum cans during formation is another application. In the can making process, a carbide punch is used to draw an aluminum cup through a series of dies to form a finished can body. The centerline position of the punch can affect the real thickness of the cans and, in prior art systems, affect the apparent thickness of the cans.
Sensing thickness using eddy currents has been accomplished by prior art systems including those which adjust circuit impedances such that the system is insensitive to distance. The system typically utilizes a classic three coil eddy current setup with a driver coil and two pickup coils. The driver coil excites the material with an AC magnetic field and the pick up coils see an impedance change caused by eddy currents induced in the material. Because this technique uses three coils, it is comparatively expensive to produce and is still sensitive to errors from lift-off (sensor--workpiece separation) if not operated within a limited range. Other systems use eddy current sensors to detect cladding thickness with a single coil, but are sensitive to lift-off errors as well. Systems with multiple frequencies have been developed to distinguish thickness changes from other effects, but dual frequencies require expensive and complex circuitry to implement.
Still others systems have measured thickness by means of eddy current devices and have used the orthogonal impedance outputs of an eddy current sensor to determine lift-off from the workpiece surface and compensate for the same. Lift-off errors have been corrected by physically moving the sensor to a specific impedance and measuring its orthogonal impedance component. However, this design has several intrinsic problems. For example, a servo system must physically position the sensor; a requirement which severely hampers the usefulness of the device as it cannot be used at high speed, cannot be placed in restrictive locations, and imposes severe environmental constraints on the use of the device. Moreover, the impedance signal components (real and imaginary) must be completely orthogonal for proper operation.
In general, known devices are capable of measuring a single parameter and are usually configured with one parameter being dependent on another. For example, the apparatus and method disclosed in U.S. Pat. No. 4,290,017 has an oscillator with an amplifier for supplying gain, a feed back loop linking the input and output of the amplifier and a two port ferromagnetic resonator connected within the loop to modulate the level and frequency of the oscillator signal in response to eddy currents induced in the surface of a workpiece sample. There is a variable attenuator connected within the feedback loop to adjust the power level of the oscillator, and an adjustable phase shifter for changing the total phase shift of the signal within the loop. There is also a probe connected to the circuit in a transition mode which is adapted to respond to the electro-magnetic field generated by the eddy currents. The probe includes a ferromagnetic crystal, with an outer circuit loop encircling the crystal and an inner circuit loop encircling the crystal orthogonal to the outer loop. The '017 device permits the measurement of changes in both the magnitude and phase of that system's signal; allowing for independent measurement of the real and imaginary parts of the impedance of the circuit.
U.S. Pat. No. 4,727,322 discloses a method and apparatus for measuring a system parameter such as workpiece thickness by means of eddy currents generated in the workpiece as a result of the proximity of a probe to the surface. The sensor in the probe measures two orthogonal components of the complex impedance. In operation, the '322 sensor is moved towards the workpiece until one component of the impedance reaches a pre-determined value and the characteristic value is measured as a value of the other component. The first component is selected during calibration as that component which is most sensitive to variation in distance between the sensor and the workpiece.
The apparatus disclosed in U.S. Pat. No. 3,358,225 details a lift-off compensation technique for eddy current testers. The '225 apparatus includes an impedance bridge having a signal generator operating at a constant frequency and voltage. The '225 apparatus is used to compare the impedance of an eddy current probe positioned in proximity to a conductive sample with a standard impedance associated with the probe to provide separated outputs of reactive and resistive components of unbalance of the impedance bridge. The '225 apparatus includes a mechanism for selecting a pre-determined portion of one of the outputs and combining it with the other output to provide a signal which is coupled to a read out device for indicating the thickness of the sample workpiece under inspection.
Eddy current non-contacting sensors also include the non-contacting displacement transducer found in U.S. Pat. No. 3,619,805 and the eddy current surface mapping system for workpiece flaw detection shown in U.S. Pat. No. 4,755,753. The eddy current flaw detection system of U.S. Pat. No. 3,718,855 and 3,496,458 rely on a single coil in a sensor probe for detection. Other single probe devices are found in U.S. Pat. No. 4,644,274 for an apparatus that supports an eddy current probe used to scan an irregular surface. The electromagnetic system found in U.S. Pat. No. 4,438,754 is used for sensing and remotely controlling the spatial relationship between a tool and workpiece. The '754 system relies on magnetic devices positioned in an opposed relationship with respect to the tool and a workpiece surface such that the magnetic flux passes between the magnetic elements through the surface.
The systems of the prior art are generally burdened by the need for multiple coils, frequencies or are not capable of simultaneous measurement of different parameters, such as material thickness and separation. It would be desirable to have an eddy current system that employs only a single coil and is capable of simultaneously measuring two parameters. The present system is drawn towards such an invention.